In recent years, next-generation displays such as ultra high definition displays (e.g., 8K4K (SHV) and 4K2K), high-frame-rate displays (e.g., 240 Hz (4× frame rate) and 480 Hz (8× frame rate)), and various 3D displays have been extensively developed. The mobility of a display-control thin film transistor (TFT) produced using amorphous silicon is about 0.5 cm2/Vs, which is insufficient for implementing a next generation high-performance liquid crystal display or organic display. Therefore, it is important to improve the performance (mobility) of a TFT in order to implement a next-generation display.
The development of a TFT used for a next-generation display was initiated to achieve a mobility of about 2 to 8 cm2/Vs. However, a mobility of 20 cm2/Vs or more, 25 cm2/Vs or more, or 30 cm2/Vs or more has been desired in recent years (see Non-patent Document 1).
A medium or small-sized display used for a tablet PC, a smartphone, and the like has also been increased in definition, and has been designed to form a peripheral circuit (e.g., driver circuit) on a substrate. A medium or small-sized display has been produced using low-temperature polysilicon (LTPS) when high mobility is required. However, since products to which such a display can be applied are limited due to an increase in cost, a technique that can inexpensively produce a TFT exhibiting a high mobility of about 30 cm2/Vs has been desired (see Non-patent Document 1).
Hosokawa et al. found an n-type semiconductor material that includes indium oxide and zinc oxide (see Patent Document 1), and various oxide films that include indium oxide and zinc oxide have attracted attention as a semiconductor material since then. In particular, since a TFT exhibiting a mobility of about 10 cm2/Vs can be produced using an amorphous oxide film that includes indium oxide, zinc oxide, and gallium oxide, an amorphous oxide film that includes indium oxide, zinc oxide, and gallium oxide has attracted attention as a material that makes it possible to inexpensively produce a TFT having high mobility used for medium or small-sized displays and large-area displays.
It has been studied to form such an oxide film using a physical film-forming technique (e.g., sputtering, pulsed laser deposition (PLD), or evaporation) or a chemical film-forming technique (e.g., sol-gel method). In particular, sputtering has been mainly studied since a film can be uniformly formed over a large area at a relatively low temperature.
It has been mainly studied to produce a TFT using an oxide semiconductor that includes indium oxide, zinc oxide, and gallium oxide so that the atomic ratio “In:Ga:Zn” is 1:1:1 or 2:2:1. However, the mobility of a TFT produced using such an atomic ratio is normally about 10 cm2/Vs, and a TFT exhibiting a high mobility of about 30 cm2V/s that is required for next-generation displays or replacing LTPS has not been produced on an industrial scale.
Attempts have been made to produce a TFT having high mobility by changing the device configuration, the TFT production conditions, and the like. However, such attempts have not been put to practical use due to various problems (e.g., it is difficult to implement production over a large area with high reproducibility due to a small thickness, a small ratio W/L, use of a stacked structure, and use of an insulating film that is not suitable for industrial production, or the off current increases, or the S-factor increases).
Attempts to improve performance by changing the compositional ratio of In, Ga, and Zn have been made by utilizing a co-sputtering method (see Patent Documents 2 and 3 and Non-patent Document 2), or utilizing various oxide targets that differ in composition (see Patent Document 4). However, a practical TFT exhibiting a high mobility of about 30 cm2/Vs that is required for next-generation displays or replacing LTPS has not been obtained.
A sputtering target that includes indium oxide, zinc oxide, and gallium oxide has also been studied, but a TFT exhibiting a high mobility of about 30 cm2/Vs has not yet been obtained (Patent Documents 5 and 6).
A sputtering target formed of an oxide sintered body differs in properties and sintering conditions appropriate for each compositional ratio. Therefore, the properties of a sputtering target and the sintering conditions appropriate for each compositional ratio have been normally studied after finding a promising material or composition. Accordingly, a compositional ratio that can achieve high performance has been searched using a co-sputtering method when improving the performance of a TFT produced using an oxide semiconductor that includes indium oxide, zinc oxide, and gallium oxide (see Patent Documents 2 and 3 and Non-patent Document 2).
However, a TFT exhibiting a high mobility of about 30 cm2/Vs has not yet been obtained even if the compositional ratio is changed using a co-sputtering method.